When materials having a surface containing soft metals, such as Zn, e.g., a galvanized steel plate, are processed or caused to slide, there is a problem in that the soft metal adheres and further is deposited on the surface of a member (e.g., a die or the like) contacting the soft metal, so that the surface quality of the workpiece is impaired.
Specifically, a hot pressing method has the following problems, for example. The hot pressing (also referred to “die quenching”) method is a technique including heating a steel plate (blank) to a temperature (usually 800 to 900° C.) in the austenite range, and then rapidly cooling and forming the same to a desired part shape with a water-cooled die. The processes from the heating of the steel plate to press processing are performed in the atmosphere from the viewpoint of cost. Therefore, in order to suppress the generation of scales due to oxidation of the steel plate, a plated steel plate, on the surface of which a plating layer mainly containing Al or Zn is formed, is frequently used as the steel plate. However, when the plated steel plate is used, particularly a galvanized steel plate is used, the number of shots increases and also Zn adheres to a pressing die, so that the die shape is deformed in an extreme case, which poses a problem in the product shape and the surface quality of the formed steel plate.
In general, a ceramic coating, such as TiN, is formed as a coating on the surface of a hot pressing die as a measure against abrasion due to rubbing with the steel plate. However, even in this case, it is hard to say that the adhesion resistance to the soft metal is sufficient.